1. Field of the Invention
This invention relates to a multi-level quadrature amplitude modulator system provided with a fading compensation means that can be applied to channels in which radio transmission path characteristics are subject to rapid fluctuation.
2. Prior Art Statement
In digital radio channels, especially land mobile radio channels, fading produces variations in the phase and envelope of received waves. Conventionally, taking into consideration an envelope fluctuation of 20 dB or more, for such channels frequency and phase modulation systems have been used in which the envelope does not contain information.
However, for further improvement in the spectral efficiency it is necessary to use a multi-level quadrature amplitude modulator system in which the amplitude also contains information.
Multi-level quadrature amplitude modulator systems have been employed conventionally for transmission paths in which fluctuation is very slow, such as microwave channels. In such applications, automatic gain control has been used to compensate for envelope distortion along the transmission path and phase-locked loops have been used to compensate for phase fluctuation.
These methods have been effective in the case of fixed microwave channels in which transmission path fluctuation is slow and gentle. However, when this fluctuation is very rapid, such as in land mobile radio channels, there have been various problems, such as:
the inability of automatic gain control to provide full compensation when envelope fluctuation is rapid and intense;
at the parts of an envelope where distortion compensation is incomplete, appropriate threshold levels for data decision cannot be set; and
because there is also intense phase fluctuation, a phase-locked loop cannot provide full compensation for phase distortion caused by fading.
Owing to such problems, when multi-level quadrature amplitude modulator systems are actually applied to land mobile radio channels, communication is possible only when a vehicle is stationary.